1. Field of the Invention
This invention relates to rotary cutting knives for cutting apparatus of the type typically used for precision cutting of multi-layer paper products such as collated pages of unbound books. More particularly, the invention relates to such knives in which multiple inserts form the blade portion of the knife and are mounted and shaped in a manner providing performance superior to similar knives of the prior art.
2. Description of the Prior Art
In the field of book making, cutting machines employing rotating cutting knives are commonly used to trim or precision cut multi-layer paper thicknesses, such as collated unbound book pages. Many of such machines employ a rotary cutting knife cutting the product and an opposed so-called "anvil knife" rotating at a surface speed matching the product feed speed and maintaining the product in proper relationship to the blades of the cutting knife to ensure precision and completeness of the cut. In such apparatus the feed stock is generally conveyed through the rotating knife array in a feed plane perpendicular to the cutting plane defined by the blade or blades of the rotary knives; the knives in turn rotate about an axis of rotation which is parallel to the feed plane.
In continuing efforts to improve the performance, life or cost of rotary cutting knives, several prior art workers have developed knives in which the cutting edges comprise series of blade inserts mounted on rotating carrier members. For example, Barbour, Jr. U.S. Pat. No. 3,990,337 discloses a cutting wheel with insertable cutting bits slidingly received in sockets and held in position by clamping blocks. The bits include straight cutting edges facing in the direction of wheel oration at an angle of about 45 degrees from a wheel radius passing through the outer tip of the blade--i.e. in each bit the cutting edge leads the rest of the bit in the direction of rotation. The cutting edges of Barbour's bit are formed by beveling an end of the bit at a predetermined angle of 20-40 degrees, and the sockets are oriented at the same angle with the cutting plane, such that the bit faces all lie in the cutting plane. Another example in the prior art is Gammerler U.S. Pat. No. 4,840,098, which discloses another cutting wheel, this one having a front face defining the cutting plane, a rear face having a truncated cone surface, and a series of rectangular blade inserts mounted on the rear face by means of screws. The blade inserts include straight cutting edges formed by grinding the insert ends at the same angle that the rear face forms with the cutting plane face, and the inserts are positioned so that bevelled faces all lie in the cutting plane defined by the front face of the cutting wheel. As with Barbour, Jr., the Gammerler cutting edges face in the rotation direction--i.e lead the body of the inserts in that direction, but in this case they form an angle of about 80 degrees with a wheel radius passing through the mid-point of the cutting edges.
All such prior art devices of which we are aware are characterized by the following:
A. The cutting edges of the blade inserts are straight;
B. The cutting edge leads the body of the insert in the direction of rotation;
C. The inserts are formed from relatively thin metal strips; and
D. The inserts are secured to a face of the cutting wheel, or in a groove cut in such face, by screws or a combination of screws and clamps.
Several shortcomings result from the design features of prior art rotary cutting knives. First, the positioning and mounting methods heretofore employed direct nearly all of the reactive cutting forces against the clamping screws or members holding the blade inserts in place. Second, the inserts have limited bending resistance because of their thin cross section, thereby making them prone to bending or breaking in use. Third, the use of straight cutting edges terminating at corners can result in unacceptable cuts if the positional adjustment of the knife or its cutting inserts is not carefully maintained to prevent the corners of the insert blades contacting the advancing paper being cut--i.e. if the blade corner strikes the paper it can cause rough cuts which are generally unacceptable in the book manufacturing industry. Finally, again because of the thin cross sections of the blade inserts, wear on the cutting edges tends to be higher than desirable, requiring frequent regrinding of the inserts to maintain sharpness.